If I purchase Sacred through gog, can I lan play via my desktop and laptop, or do I need to purchase the game twice?

If you own both machines you are going to use for LAN play, then no, you don't need to purchase it twice. GOG allows you to install the game as many times as you want on as many machines as you own.

I'll add the the required CD keys appear to only be for Open/Closed Multiplayer. They don't come up required for a LAN game.